Adaptive nonlinear image enlargement using wavelet transform coefficients

#The present invention relates to the field of image processing; more particularly, the present invention relates to edge preserving image interpolation with wavelets.

#Resizing of digital images is often performed in digital imaging processing and becomes more important in networks environments that include devices with different dpi resolution. While aliasing and moiré artifacts are the main problems in image reduction, enlargement of images has to deal with the problem of how to introduce high frequency components in order to have the image, in particular edges, not appear too smooth or too blurred. A typical method for enlargement of images is the use of an interpolation filter. This filtering incorporates information from neighboring pixels in order to predict an interpolation value. Commonly used filters as, e.g., in a popular photo-image manipulation application, are bilinear or bicubic interpolation filters. With the use of those filters a perfect step edge cannot be interpolated to produce a perfect step edge at a higher resolution. The interpolated edge will always look a bit blurred.

#A standard method for image interpolation is polynomial interpolation. Depending on the degree of the interpolating polynomial (e.g., linear, quadratic, cubic, etc.), the image looks more or less smooth. The most commonly used technique is referred to as cubic interpolation. An advantage of polynomial methods is their simplicity since they are based on global linear filtering techniques. A disadvantage is that it is not possible to perform an adaptive interpolation, thereby resulting in edges typically being oversmoothed. This is a significant disadvantage in enlargement of documents.

#The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only.

#FIG. 1 illustrates two-dimensional extension of one-dimensional nonlinear interpolation methods;

#FIG. 2 is a flow diagram illustrating the process for embedding interpolation within an inverse wavelet transform;

#A method and apparatus for adaptive nonlinear image enlargement using wavelet transform coefficients is described. Wavelet transform coefficient naturally divide the image into smooth and edge parts. The present invention performs interpolation in the wavelet domain and uses the information in lowpass and highpass coefficients to automatically perform a smooth interpolation in smooth regions and to predict sharp edges in areas of high frequency.

#The technique described herein is a modified approach to the idea of nonlinear adaptive interpolation by performing adaptive interpolation in the wavelet domain. In one embodiment, this preserves significant sharp edges that are above the noise level (characterized by, for example, a standard deviation a of wavelet coefficients as described below), where edges can be isolated, including short edges, and does not enhance noise pixels. An edge sensitivity parameter k is eliminated from the prior art technique described above and a parameter p that determines the metric distances are measured in is utilized. Moreover, the technique may adapt the algorithm to arbitrary wavelet systems and transforms. In one embodiment, the technique is combined with wavelet-based denoising and enhancement techniques, and is therefore a useful addition to a wavelet sharpening and smoothing (“WSS”) technique. It is also applicable to JPEG 2000 (J2K) compressed images.

#In the following description, numerous details are set forth, such as distances between components, types of molding, etc. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.